Add StringValueDirectAnswer interface.

This PR creates the `StringValueDirectAnswer` interface, which
describes a direct answer sourced from a string (or string list) field
in the Knowledge Graph.

J=SLAP-2315
TEST=manual

I made a few code changes locally which are not included in this PR. These
changes allowed me to generate a switch statement on `fieldType`. I added
cases for `single_line_text` and `multi_line_text`. These cases were hit as
expected when getting direct answers for string and string list fields. In
these switch statements, I saw the IDE correctly inferring that the DA was a
`StringValueDirectAnswer`.

docs/search-core.builtinfieldtype.md

@@ -17,4 +17,6 @@ export declare enum BuiltInFieldType
 |  Member | Value | Description |
 |  --- | --- | --- |
 |  Address | <code>&quot;address&quot;</code> |  |
+|  MultiLineText | <code>&quot;multi_line_text&quot;</code> |  |
+|  SingleLineText | <code>&quot;single_line_text&quot;</code> |  |
 

etc/search-core.api.md

@@ -96,7 +96,11 @@ export interface BaseSearchConfig {
 // @public
 export enum BuiltInFieldType {
     // (undocumented)
-    Address = "address"
+    Address = "address",
+    // (undocumented)
+    MultiLineText = "multi_line_text",
+    // (undocumented)
+    SingleLineText = "single_line_text"
 }
 
 // @public

src/models/searchservice/response/BuiltInFieldType.ts

@@ -16,6 +16,6 @@ export enum BuiltInFieldType {
   // Hours = 'hours',
   // Decimal = 'decimal',
   // RichText = 'rich_text',
-  // SingleLineText = 'single_line_text',
-  // MultiLineText = 'multi_line_text'
+  SingleLineText = 'single_line_text',
+  MultiLineText = 'multi_line_text'
 }

src/models/searchservice/response/StringValueDirectAnswer.ts

@@ -0,0 +1,11 @@
+import { BuiltInFieldType } from './BuiltInFieldType';
+import { FieldValueDirectAnswer } from './FieldValueDirectAnswer';
+
+/**
+ * A direct answer whose source is a string or string list field in the knowledge graph.
+ *
+ * @public
+ */
+export interface StringValueDirectAnswer extends FieldValueDirectAnswer<string | string[]> {
+  fieldType: BuiltInFieldType.SingleLineText | BuiltInFieldType.MultiLineText
+}